Sinkholes are appearing with increasing frequency in many parts of the world, often with little warning. The ground can remain stable for years while water gradually erodes the rock below, creating large underground cavities. When the surface layer can no longer support its own weight, it collapses, sometimes taking cars, buildings, or sections of road with it.

In Turkey, a large sinkhole opened near the city of Ka, roughly 65 miles from the city center. It measures about 100 feet wide and 50 feet deep. Officials placed warning signs and fences around the area, as the ground may continue to settle. The region already has around 700 recorded sinkholes, and new ones are appearing in nearby towns and villages as well.

Geologists say the causes are mainly underground. For a long time, these cavities were filled with water, which helped support the ground from below. But heavy pumping of groundwater for farming, combined with reduced rainfall, has lowered water levels significantly. As the water disappears, the ground above loses support and can collapse suddenly.

The region is also located in a fault-controlled basin, meaning the ground shifts naturally over time. Small vibrations from earthquakes can trigger a collapse where the surface is already weak. In some places, underground streams carry loose soil deeper, expanding the empty spaces further.

A similar event occurred at a mining site in Chile’s Atacama Desert. A sinkhole appeared near an underground copper mine, initially about 80 feet wide before expanding to nearly 105 feet. No one was injured, and no equipment was damaged, but workers stopped underground operations while geologists investigated the cause.

In 2010, a sinkhole opened in the middle of Guatemala City. It was roughly 60 feet wide and about 20 stories deep, and it swallowed a three-story building. The collapse likely resulted from a combination of long-term weakening, possibly from a damaged sewer or storm drain, and heavy rainfall from Tropical Storm Agatha, which may have been the final trigger. Specialists attempted to stabilize the hole by filling it with rock, dirt, and debris, but its size made the operation difficult.

One of the largest known sinkholes is in southern China, called Xiaozhai Tiankeng. It drops to a maximum depth of about 2,000 feet and is roughly 1,760 feet across. It formed over thousands of years as rainwater seeped into the limestone, creating tunnels and caves. The land above eventually collapsed, possibly in stages, leaving two separate levels. The bottom is largely cut off from sunlight, which has created a cool, humid environment. Researchers have identified more than 200 plant species there, including ancient ginkgo trees and large ferns, along with animals such as clouded leopards.

Sinkholes can also form through human activity. Construction, drilling, pipes, and pumping water from underground can weaken the land, and a period of heavy rain can then trigger a collapse. One of the most active areas is around the Dead Sea, where the ground contains large amounts of salt that dissolve quickly in water. New sinkholes open frequently there, and some have injured tourists and researchers.

In Peru, a different kind of ground feature has drawn attention. Known as the Band of Holes, it consists of thousands of small pits stretching in rows along a mountainside. The site was first noticed by an aerial photographer in 1931. For decades, scientists could not explain its purpose. Some speculated it was a defense line, a series of graves, or an irrigation system, but none of these theories held up.

Archaeologists later found traces of corn and wild plants used for weaving near the site, along with pieces of pottery dating to the 15th century. That period corresponds with the Inca conquest of the Chincha people, a pre-Columbian group living on the southwestern coast of present-day Peru. The Incas did not use money as we understand it, but they needed to collect tribute from conquered peoples. Researchers now believe the pits were used as a measurement system. Families filled them with goods such as corn, cotton, and peppers, and Inca officials collected the items and transported them to a nearby administrative center. In effect, the Band of Holes was a form of tax collection.

The site is not far from the Nazca Lines, a series of geoglyphs in southern Peru that include hundreds of geometric shapes, straight lines, and images of animals and plants. They are attributed to the Nazca people, who flourished from around 1 to 700 CE. Because the shapes are best seen from above, they were only widely identified after the development of aviation in the 1930s. Various theories have been proposed, including connections to astronomy, but none has been proven. Some researchers suggest the geoglyphs were linked to rituals intended to bring rain in a region that receives very little precipitation.

Sinkholes have also caused serious incidents in urban settings. In Kuala Lumpur in August, a woman walking down a street was swallowed by a 26-foot-wide hole that opened beneath her. More than a hundred rescuers searched for her over several days. Divers were sent into the sewers, but strong currents and poor visibility hampered the effort. A second hole opened in the same neighborhood days later, and local businesses reported a sharp drop in visitors.

In Naples, a sinkhole opened on a quiet street earlier this year, swallowing two cars. One of the cars was parked, but the other had people inside. The passengers escaped with minor injuries. The mayor attributed the event to years of neglect of the city’s sewer system.

Florida, sometimes called Sinkhole Alley, has seen many such events. In Highland City, about 40 miles east of Tampa, a 60-foot-deep sinkhole appeared in a resident’s front yard, just inches from a child’s bedroom. The house was undamaged. A more serious incident occurred in Hillsborough County in 2013, when a sinkhole opened beneath a one-story home at night. One resident was killed. Fire officials judged the ground too unstable for a full rescue operation. The county later purchased the property and surrounding land to prevent future construction. The sinkhole reopened twice more, but the area was empty both times.

Florida is especially vulnerable because most of its ground is made of carbonate rocks such as limestone, which erode easily. The state also experiences cycles of drought followed by heavy rain, and rising sea levels are pushing groundwater higher. Local governments have tried to manage the situation by rerouting underground water and reinforcing vulnerable areas, but sinkholes cannot be entirely prevented.

Sinkholes are not always destructive. Some have become tourist attractions or habitats for unique ecosystems. The Dragon Hole, near the Paracel Islands, is a deep underwater sinkhole about 984 feet deep, believed to be the deepest blue hole in the world. Scuba divers and snorkelers visit it for its marine life.

In Brazil, the Hole of Macaus is an island sinkhole about 328 feet deep. It serves as a home to thousands of macaws, with researchers estimating around 60 breeding pairs. The birds use the rocky walls as shelter.

In Oman, the Bimmah Sinkhole is located in a dry, rocky desert and contains a turquoise pool of water. Locals once believed a meteorite created it, and the name reflects that legend, but it actually formed when limestone eroded over time. The hole is about 135 feet wide and 65 feet deep, and it has been turned into a park.

Underground cavities are not always the result of water erosion. In southern Brazil, researchers have found more than 1,500 caves that appear to have been dug by enormous prehistoric animals. These structures, called paleoburrows, have smooth walls and rounded ceilings that do not resemble natural erosion. Scientists believe they were created by giant ground sloths and large armadillos, which could weigh nearly 9,000 pounds. Analysis of scratch marks suggests the animals worked in groups over long periods. The tunnels may have been used as shelter from climate changes, since their large bodies made it difficult to regulate temperature.

These animals went extinct around 11,000 years ago. Most mammals heavier than about 2 pounds disappeared at that time. Researchers generally point to a combination of human hunting and climate change. The end of a cold period altered vegetation, and large herbivores lost their food sources, followed by the predators that relied on them. Large animals also reproduce slowly, which made it harder for their populations to recover.

The conditions that allowed such giants to exist do not apply to modern mammals. Higher oxygen levels in the distant past may have supported larger insects, and the buoyancy of water allowed whales to grow far larger than any land animal. The blue whale, at up to 330,000 pounds, is the largest mammal ever known. On land, that size would be a serious health risk.

Some modern sinkholes have also been linked to human decisions. A beach in an unnamed location saw a boy trapped up to his neck in sand after a hole he was digging collapsed. Emergency services freed him without serious injury. Experts note that sand is unstable when disturbed and recommend never digging a hole deeper than the knee level of the smallest person in a group, and always filling holes before leaving the beach.

Natural hazards are not limited to sinkholes. Hailstorms can cause significant damage to buildings, cars, and crops, and can be dangerous to aircraft. Rip currents, which move at speeds up to eight feet per second, can pull swimmers away from shore. The correct response is to swim parallel to the shore rather than directly against the current. Landslides can occur after heavy rain or earthquakes, and fire whirls can form during wildfires, reaching temperatures up to 2,000 degrees Fahrenheit.

Caves themselves can present hazards. Cathedral Caverns in Alabama has a large entrance and notable rock formations, but only about two miles have been mapped. During filming of a 1984 movie, 17 cast and crew members experienced carbon monoxide poisoning and required hospital treatment. Moaning Cavern in California produces a breathing-like sound from water dripping through openings, and researchers have found remains of more than 100 prehistoric people who fell into a 410-foot chasm there.

Some caves are biologically significant. Movile Cave in Romania has been isolated from the outside world for about 5.5 million years. The air contains roughly half the oxygen of normal air, with high levels of carbon dioxide and hydrogen sulfide. Despite these conditions, researchers have identified 48 species, 33 of which are unique to the cave. Most are small, colorless, and eyeless, and they survive through chemosynthesis, a process in which bacteria convert chemicals into food without sunlight.

Other caves have produced notable archaeological findings. Lovelock Cave in Nevada was used as a burial site, and some remains found there are larger than average human bones. According to local legend, a tribe of red-haired giants lived in the area and were buried in the cave. Some scientists suggest the reddish hue may be the result of environmental factors, and the larger bones could belong to prehistoric animals. The cave has also yielded finely crafted baskets and duck decoys made from tule fibers.

The Great Sphinx of Giza, carved from a single piece of limestone about 4,500 years ago, is known to conceal a system of tunnels and chambers. Researchers have found entrances near the statue’s feet, but active excavation is considered risky because it could damage the structure. Some theories suggest the passages could lead to hidden repositories, but there is no evidence to support those claims.

Underground spaces are not always natural. In Turkey, a resident discovered a hidden passage in his basement in 1963, leading to an underground city that could have housed about 20,000 people. The complex included a chapel, a school, and stables, and it is connected to a network of tunnels and other underground towns.

A similar discovery was made in Vietnam, where a logger found a cave entrance in the jungle in 1991. He returned years later with scientists, and the cave, called Son Doong, is now considered the largest in the world. Its main passage reaches 660 feet in height, and it contains its own jungle, rivers, and unique microclimate, formed where sections of the ceiling have collapsed to let in sunlight. The cave has been open to tourists since 2013, with expeditions lasting several days.

In Indonesia, researchers found the remains of a small human ancestor species in a cave on the island of Flores. The female skeleton stood about three and a half feet tall, and the species, Homo floresiensis, lived between roughly 190,000 and 50,000 years ago. It is not clear whether they ever encountered modern humans.

Caves have also provided insights into human biological rhythms. In 1962, a French geologist named Michel Siffre spent two months in a cave in the French Alps without access to natural light or time cues. He found that his internal clock drifted from the standard 24-hour cycle. Later experiments suggested that in the absence of time cues, humans may shift to a longer cycle of activity and sleep. In 2021, a Spanish athlete, Beatriz Flamini, spent 500 days in a cave near Granada with no contact with the outside world, as part of an experiment on isolation. She reported losing track of time and noted that she did not want to leave before finishing a book she was reading.

The moon has also been compared to underground environments in terms of isolation. Lunar lava tubes, formed when molten rock cooled and drained, could provide shelter for future astronauts. The largest known one is about 427 feet in diameter and could accommodate an entire football field. Scientists have also studied the moon’s geology and history, noting that its crust varies in thickness and that water ice exists in permanently shadowed regions near the poles.

Weather records offer a different kind of comparison. In Hawaii, a location recorded 331 consecutive days of measurable rainfall from 1939 to 1940. Another site, Hanumo Maki, recorded 881 consecutive days of rain from 1913 to 1916. These periods were possible because of the region’s tropical rainforest conditions and the effect of mountains forcing moist air upward.

Certain weather phenomena are rare and depend on specific conditions. Fire rainbows, or circumhorizontal arcs, only form when sunlight passes through cirrus clouds at a high angle. In London, the sun never reaches the required height, so the effect never occurs there. Underwater crop circles, found off the coast of southern Japan in 1995, were eventually explained in 2011 as the work of a small puffer fish, which arranges sand and shells to create a nest. Frost flowers, thin sheets of ice that form on plant stems, appear only under specific cold, damp conditions when water inside the stem freezes and expands, pushing out delicate layers of ice.